Vanadium (IV) oxide (VO2) is a technologically important material based upon its ability to undergo a fully reversible metal-to-semiconductor phase transition. The conversion of the low temperature monoclinic phase VO2(M) to the high temperature rutile phase [1] VO2(R) is associated with significant changes in electrical conductivity [2] and optical properties [3] in the near-IR region. VO2(R) is a semi-metal, reflecting a wide range of solar wavelengths. VO2(M) is a semiconductor and reflects considerably less energy. VO2 having such a reversible metal-to-semiconductor phase transition is said to be thermochromic.
These properties have led to suggestions of using VO2 in data storage [4 & 5], infrared modulators [6] and intelligent window coatings, i.e. windows which respond to an environmental stimulus [7 & 8]. An intelligent window coating operates by a thin film of thermochromic material on an exterior window modifying the window's reflectance properties as a function of the outside ambient temperature [9]. The solar radiation that is not able to pass through the window when it is in its darkened state must be either reflected or absorbed. Ideally, in a cooling dominated application, the window would pass all or part of the visible radiation incident on the window and reflect the majority of the Sun's near infrared radiation. Incident solar radiation, that is not transmitted, is absorbed. Absorption will cause significant heating of the window if left to stagnate in a coloured state under conditions of high irradiance. Temperature rises in the window will give rise to a radiant heat source adjacent to the room, potentially leading to thermal discomfort, and will impose additional demands on the temperature stability of the materials used in the smart window. Effective thermochromic window coatings would respond to this heating by increasing their reflectance and compensating for the increased heating by reflecting more heat away. Such intelligent coatings could be used in applications such as car windscreens, sunscreens and greenhouses. Such coatings have the potential to provide savings in energy costs (e.g. power to air conditioning units), improved building environments, and environmental benefits (e.g. reduced CO2 emissions).
However, in order for VO2, coatings to be practically useful in these applications, the phase transition temperature between the monoclinic phase and the rutile phase (also referred to as the thermochromic switching temperature) must be lowered. For example, for intelligent window coatings the thermochromic switching temperature should be just above room temperature, e.g. about 25-30° C., although ambient climatic conditions will affect the precise choice. Other applications, such as for night vision apparatus, require VO2 having a thermochromic switching temperature below room temperature, i.e. below 25° C.
Unfortunately, the thermochromic switching temperature of VO2 itself is 68° C., meaning that unmodified VO2 is not ideal for the above mentioned applications. Researchers have therefore developed techniques for reducing the thermochromic switching temperature of VO2, the most efficient of which has been doping tungsten ions into the VO2 lattice using sol-gel [9 & 10] and physical vapour deposition methods [11 & 12]. However, these known techniques are slow, are not compatible with large area glass manufacture and are unsuitable for incorporating into conventional float glass production lines as they require off production line manufacture, such as cutting the glass before deposition.
Reference 13 discloses the use of APCVD for producing films of thermochromic transition metal-doped vanadium (IV) oxide. Films of tungsten-doped vanadium (IV) oxide were obtained with tungsten doping up to 3.1% and with transition temperatures down to 5° C. However, the films obtained in reference 13 were yellow-brown in colour, which is not an ideal colour for use in intelligent window coatings.
An object of the invention is therefore to provide films of thermochromic vanadium (IV) oxide, in particular thermochromic transition metal-doped vanadium (IV) oxide, having improved colouring, particularly for use in intelligent window coatings (e.g. coatings for architectural glass), while maintaining the thermochromic properties of the films. A further object of the invention is to provide methods for production of the films of the invention having improved reproducibility compared to those known in the art.